I am not sure what you have for tools, but at work I get this quite often. I use a small cutting wheel on a dremel and make a slot in the head of the screw and use a flat screw driver. My last option is drilling off the head with a good drill bit but this can still make it difficult to grab the remaining portion of the screw but at this point it is typically loose in the threads.

DOG is GOD spelled backwards.
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